Along with the rapid development of electronic, communication and computer industries, there are remarkable improvements in camcorders, cellular phones, notebooks and so on. Thus, the demand for lithium secondary batteries is increased day by day as a power source to drive such portable electronic communication devices. In particular, researches and studies are actively progressing not only in Korea but also in Japan, Europe and USA on lithium secondary batteries as an environment-friendly power source that may be applied to electric vehicles, uninterruptible power supplies, electromotive tools and satellites.
Lithium cobalt oxide (LiCoO2) has been frequently used as a cathode active material of a lithium secondary battery, but in these days, lithium nickel oxide (Li(Ni—Co—Al)O2) and lithium composite metal oxide (Li(Ni—Co—Mn)O2) are also used as other layered cathode active materials. In addition, spinel-type lithium manganese oxide (LiMn2O4) and olivine-type ferric phosphate lithium compound (LiFePO4) with low price and high stability are increasingly consumed.
However, a lithium secondary battery using a lithium cobalt oxide, a lithium nickel oxide or a lithium metal composite oxide exhibit insufficient safety, particularly in thermal stability and overcharging characteristics, though it has basic battery characteristics. As an improvement, there are introduced various safety means such as shutdown functions, additives of electrolyte and safety device like PTC, but such safety means are designed under the condition that cathode active material is not filled to a high level. Thus, if the cathode active material is filled to a high level so as to meet the demands on high capacity, various kinds of safety means tend to be operated inappropriately, and also the safety may be deteriorated.
Also, a spinel-type lithium manganese-based battery has ever been applied to cellular phones at one time. However, in the recent cellular phone market requiring superior functions for the first time, its advantages such as low price and high safety are not utilized due to its gradually deteriorated energy density.
Also, an olivine-type ferric phosphate lithium compound has very low electron conductivity in spite of advantages such as low price and high safety, so excellent battery characteristics are not expected. Also, an average operating potential of the olivine-type ferric phosphate lithium compound is low, which does not satisfy the demands on high capacity.
Accordingly, various researches have been performed to solve the above problem, but an effective solution is not yet proposed.
For example, Japanese Laid-open Patent Publication No. 2001-143705 discloses a cathode active material in which a lithium cobalt oxide and a lithium manganese oxide are mixed. However, this material is just obtained by simply mixing a lithium manganese oxide with excellent safety, so it does not improve characteristics sufficiently.
Also, Japanese Laid-open Patent Publication No. 2002-143708 proposes a cathode active material in which lithium nickel composite compounds with different compositions are provided in two layers. However, since a cathode active material having two layers of lithium nickel composite compounds with different compositions is applied, it cannot be considered as fundamentally sufficiently improving the safety caused by overcharging.
Japanese Laid-open Patent Publication No. 2007-012441 discloses a cathode having two or more cathode active material layers to improve the overcharging characteristics, where an olivine-type ferric phosphoric lithium oxide or a spinel-type lithium manganese oxide is added to the layer contacting with a cathode current collector. However, though the improvement in overcharging characteristics is expected, such oxide layers are formed with a thickness not greater than its average particle size, namely in a level of several micrometers, and conductive material or auxiliary conductive material is not included, so it cannot be considered as having sufficient high-current discharging characteristics.
Japanese Laid-open Patent Publication No. 2006-318815 discloses a technique for coating a surface of secondary particle with lithium salt or lithium oxide so as to improve durability of a lithium nickel oxide. However, it is difficult to coat the entire surface of individual cathode active material secondary particles, so its improvement is not remarkable. Also, since a dry-coating process and a wet-coating process should be added, the productivity is greatly deteriorated.
Japanese Laid-open Patent Publication No. 2006-19229 proposes to coat a surface of secondary particle with lithium cobalt zirconium oxide for the purpose of improving the safety of lithium nickel oxide with weak safety. However, since a dry-coating process should also be applied to coat the surface of lithium nickel oxide secondary particle with lithium cobalt zirconium oxide, the productivity is deteriorated though its effect is remarkably improved.
Thus, it is urgent to develop a cathode active material having excellent safety while keeping excellent battery characteristics, and a method for preparing such a cathode active material with an excellent productivity.
[Disclosure]
[Technical Problem]
The present invention is designed to solve the problems of the prior art, and therefore it is an object of the present invention to provide a cathode active material capable of greatly improving safety, particularly thermal stability and overcharging characteristics, without deteriorating basic characteristics of the cathode active material itself, and to provide a method for preparing such a cathode active material with excellent productivity and reproduction.
[Technical Solution]
In order to accomplish the above object, the present invention provides a cathode active material for a lithium secondary battery, which includes a lithium metal oxide secondary particle core formed by agglomerating lithium metal oxide primary particles; and a shell formed by coating the secondary particle core with barium titanate and metal oxide.
In relation with the background art, there were attempts to coat a surface of lithium metal oxide into a film shape for the purpose of improving reliability of high temperature characteristics of the lithium metal oxide, but all attempts made in the background art were directed to reforming the surface using electrically inactive material, so the inventors found that the surface reformation may give bad effects such as the reduction of unit capacity of the cathode active material itself or the deterioration of high rate characteristics when the cathode active material is used for making a battery.
However, the inventors also found that the cathode active material for a lithium secondary battery according to the present invention may improve thermal stability while keeping excellent electric characteristics by coating a lithium metal oxide secondary particle with barium titanate and metal oxide. Barium titanate changes its crystal structure at about 125° C. to greatly increase an electric resistance as mentioned above, thereby improving thermal stability of the cathode active material.
Also, the sell made of the cathode active material according to the present invention may further include olivine-type ferric phosphate lithium oxide and/or conductive material, selectively. If overcharging occurs, the olivine-type ferric phosphate lithium oxide may limit discharge of lithium from lithium metal oxide corresponding to the core to improve the safety of the cathode active material against overcharging, and the conductive material may improve discharging characteristics of the cathode active material.
In another aspect of the present invention, there is also provided a method for preparing a cathode active material for a lithium secondary battery, which includes (S1) firing metal hydroxide and lithium salt to make a lithium metal oxide secondary particle core in which lithium metal oxide primary particles are agglomerated; (S2) dry-coating the core with barium titanate and metal oxide to form a shell on an outer surface of the core; and (S3) thermally treating the resultant material.
The method for preparing a cathode active material for a lithium secondary battery according to the present invention adopts the dry-coating process, so it allows coating the surface of cathode active material with barium titanate and metal oxide corresponding to the shell with excellent reproduction in an efficient way while keeping a conductive passive film existing on the surface of the cathode active material corresponding to the core.
In the method for preparing a cathode active material according to the present invention, the thermal treatment of the step (S3) may be conducted at 300 to 600° C. for 4 to 12 hours.
The cathode active material for a lithium secondary battery may be used for making a cathode of a lithium secondary battery, or a lithium secondary battery having such a cathode.